A trainer I used once, used this method, as does most everyone I come across. The trainer was really big on making the horse she's training as soft as possible. In the beginning the horse needed quite a bit of force, as she had very little ground manners. As time went on, the amount of wiggle needed in the rope was an indication of her progress. It made a lot of sense for training purposes. That was not the ONLY method of backing that she was taught either, there were at least 3 that were included in her training.